Our aim has been to reveal the topographic distribution of neurotransmitter receptors on nerve and muscle cells, while relating this distribution to the development and function of synapses. To this purpose, we have used alpha-bungarotixin as a specified probe for the visualization and quantitation of nicotinic acetylcholine receptor sites. Our recent work has emphasized a model system for studying the neural control of nicotinic acetylcholine receptor distribution on skeletal muscle fibers. This system is based on the finding that embryonic and clonal nerve cells produce a factor which induces aggregation of receptors on cultured skeletal myotubes. We have now provided electron microscopic evidence for the involvement of cytoskeletal structures and extracellular matrix structures in the formation and maintenance of receptor aggregates. We have also provided morphological and kinetic evidence that laminin, a basement membrane glycoprotein, may be involved in the induction of receptor aggregates by neuronal factors.